Is there any way to fill the whole back of this mesh?
Hoo boy.
Ok so before I suggest something to solve this at this stage, id like to really reconsider your workflow in modeling: Fill first, extrude later. Also: do not intersect geometry (non-manifold meshes)

For this case i would say that maybe if you used the "Grid Fill" feature you could fill the faces to make sense on automatic (space -> search: "grid fill").
Otherwise theres always the soul-crushing way of doing it by hand: quad by quad (have fun)

At this stage id just trash what you did here, and restart the whole model with the elements already filled and then extrude them. It doesnt look like too much work.